For many years, petroleum has been recovered from subterranean reservoirs through the use of drilled wells and production equipment. During the production of desirable hydrocarbons, such as crude oil and natural gas, a number of other naturally occurring substances may also be encountered within the subterranean environment.
Many undesirable downhole products must be managed during the production of hydrocarbons. For example, scale, paraffins, fines, sulfur, heavy oil tar by-products, hydrogen sulfide, and water blocks commonly accumulate in and around the formation, well casing, production tubing and recovery equipment. Alternatively, it may be necessary to remove injected fluids from the near wellbore area, such as drilling fluids, cement filtrate, kill fluids, polymers and water blocks. To maintain an efficient recovery of petroleum, it is frequently necessary to clean or remove these accumulations and deposits.
The removal of unwanted deposits from the wellbore and production equipment is generally referred to as “remediation.” In contrast, the term “stimulation” generally refers to the treatment of geological formations to improve the recovery of hydrocarbons. Common stimulation techniques include well fracturing and acidizing operations. Well remediation and stimulation are important services that are offered through a variety of techniques by a large number of companies.
The effectiveness of well remediation and stimulation operations is greatly affected by the chemical agents used during these services. Operators have discovered that microemulsions can offer an advantageous vehicle for delivering solvents or other treatment chemicals to a targeted treatment area. Microemulsions are characterized as either water-in-oil (an aqueous phase is dispersed in an oil phase) or oil-in-water (an oil phase dispersed in an aqueous phase) mixtures. Microemulsions comprise droplets having a diameter in the range of about 1-100 nm. These systems appear clear or transparent to the human eye compared to conventional emulsion systems which have droplet sizes >400 nm in diameter and a cloudy appearance. Microemulsions are thermodynamically stable and exhibit no phase separation.
The separation of the oil and water phases depends on the creation of an interface of surfactant molecules between the water and oil molecules. Nonionic surfactants are often classified according to the hydrophile-lipophile balance (HLB) system. The HLB value of a surfactant is a measure of the degree to which it is hydrophilic or lipophilic, determined by calculating values for the different regions of the molecule. Originally, the HLB value was defined as a relative measure of the ratio of the weight percent ethylene oxide (EO) in a given nonionic surfactant.
Surfactants with lower HLB values are typically used for making oil-external (water-in-oil) microemulsions, while surfactants with higher HLB values are used for making water external (oil-in-water) microemulsions.
The prior art includes many examples that show that surfactants with an HLB generally greater than about 8 will produce oil-in-water emulsions, while surfactants with an HLB of about 3 to 8 will produce water-in-oil emulsions.
U.S. Pat. No. 7,380,606 (Pursley, et al.; Jun. 3, 2008) titled “Composition and Process for Well Cleaning” states that water-in-oil microemulsions should be prepared using surfactants with an HLB value of between 3 and 8, while oil-in-water microemulsions are prepared with surfactants having an HLB of 8 to 18.
An article by D. F. Brost and J. Silveira titled, “Surfactants Assist Water-in-Oil Monitoring by Fluorescence” in the October 2008 online issue of World Oil states, “Surfactants with an HLB less than 10 stabilize water-in-oil emulsions. Surfactants with an HLB greater than 10 stabilize oil-in-water emulsions. Within that group, surfactants with an HLB of 12-15 act as detergents and are commonly used to remove oil from solid surfaces.”
U.S. Pat. No. 3,919,411 titled, “Injectible Adjuvant and Compositions Including Such Adjuvant”, Nov. 11, 1975 by Glass, et al. states, “Where surfactants are used for dispersing oil in water, the final HLB number should be above 6. In the case where surfactants are used of the type for dispersing water in oil, the final HLB number should be below 6.”
In a presentation given to the Midwest Chapter of the Society of Cosmetic Chemists in 2004, Uniqema reported that for making water-in-oil emulsions, surfactants with HLBs of 4-6 should be chosen, while surfactants with HLBs of 8-16 should be used when making oil-in-water emulsions.
In direct contrast to the prior art, the present invention describes the surprising discovery that high HLB surfactants can be used to produce water-in-oil microemulsions that find use in oilfield and well treatment applications.
Although a number of compounds and techniques are known in the prior art, there is a continued need for more effective microemulsions for remediation stimulation services. There is a particular need to identify new surfactants and surfactant mixtures that can be used to prepare stable microemulsions.